The Absorption Spectrum of Caesium Hydride

A ${}_{}{}^1{}_{}{}^{}\Sigma \to {}_{}{}^1{}_{}{}^{}\Sigma$ molecular spectrum of caesium hydride has been photographed in absorption with a three meter glass prism spectrograph. The spectrum is of the many lines type and the observed part lies between 4550A and 6250A. In the longer wave-lengths the bands are masked by a system of ${\mathrm{Cs}}_2$ bands. The CsH system consists of 31 two-branch bands falling into three $v^{\prime }$ progressions. There is an additive uncertainty in the $v^{\prime }$-numbering. The rotational and vibrational constants have been calculated for the two electronic states. ${B_v}^{\prime }$ and $\Delta G^{\prime }$ show the anomaly characteristic of alkali hydrides, rising with increasing $v^{\prime }$ to a maximum and then falling off. The heats of dissociation are 1.10 and 1.96 volts for the excited and ground states. Potential energy curves are drawn and it is shown that Mulliken's explanation for the anomalous behavior of the excited states of alkali hydrides fits nicely in the case of CsH.